This invention concerns the manufacture of synthetic iron oxide. More particularly, it concerns the minimizing of the soluble iron in the liquid effluent from a typical scrap iron process. The utility of the invention resides in (a) increased efficiency of the process and (b) the reduced amount of soluble iron discharged to the environment.
The production of pigmentary iron oxides by the scrap iron process is well known and is described in the Martin patent, U.S. Pat. No. 2,939,767. This patent also discloses some of the names used for pigmentary yellow iron oxide, such as alpha ferric oxide hydrate. We prefer to use the simple designation of "iron oxide" for the yellow iron oxide product prepared in this disclosure.
In a typical scrap iron process, such as the Penniman-Zoph process outlined in the Martin patent, and well known in the pigment industry, the liquid effluent obtained at the completion of the reaction contains residual iron ion as Fe.sup.++. One prior method of utilizing this residual iron has been the uncontrolled addition of hydroxide iron to the scrap tank, with scrap present. This method forms a mixture of products, such as ferrous hydroxide and ferric hydroxide, which tends to lower the quality of the finished oxide product. Another way of treating the residual iron ion is to remove the mixture of iron oxide and accompanying effluent from the reaction tank and filter the mixture. The filtrate is then treated with alkali and aeration, with a typically undesirable product obtained, due to low concentration of ferrous iron, short retention time, and absence of seed crystals. Since dumping of the iron ion-containing effluent to the sewer or nearby stream is prohibited and since prior attempts to utilize the low concentration of soluble iron give low-grade products, the industry has sought alternative methods of utilizing the effluent stream and obtaining desirable iron oxide.